There are filters that allow you to see switch between "since last visit" and "all ads".

Click Buy & Sell link at the top of the page
Select a category on the left side of the page (ie. Airplanes)
Just above the adds you will see: Set Your Browsing Filters Show Filters / Hide Filters
Select to Show Filters and change the setting to view All Ads

This is an immediate toggle so you can switch back and forth quickly to suit your preference.